Help me out in this query

Hi,
I want a details like, take a person id, his billng details available in abc table. he may not be pay bill every month. he can pay with 3 months gap or 4months gap etc..or continusly.
my condition is I want a list of person ids with his last payment bill date and previous bill date should be greater than 90days.

I have taken aggregation function like maximum date(last bill date) and second maximum(previous bill date)those bill payment dates of that person id, but how to choose those days should be greater than 90days.
is there any function like add 90 days to a field???